This invention relates to the field of human eye examination and more particularly to apparatus and method for automatically measuring refractive error in the vision of a human patient.
Light entering a human eye is refracted (bent) by the cornea and lens of the eye to converge and focus to some location behind the lens. If light from a distant object (theoretically at infinity) focuses on the retina of the eye then, if there is no distortion due to astigmatism (discussed momentarily), the eye is considered free from refractive error. When this is the case, the person can see distant objects clearly. In order to view a near object clearly, i.e., cause light from the object to converge and focus on the retina, it is necessary that the curvature of the lens of the eye be increased. This is achieved by the action of a muscle and is called "accommodation".
If light from a distant object converges to a point in front of the retina, the distant object is not seen clearly by the person and the person is said to be nearsighted. This "refractive error" of the eye can be corrected by an eye glass or lens which causes light from distant objects to diverge slightly as it passes through the lens. Such a lens is considered to have "negative refractive power." The light then passes through the lens of the eye and focuses on the retina thus enabling the person to clearly see the distant object.
If light from a distant object passes through the lens of the eye and converges toward a point behind the retina of the eye, then the person is said to be farsighted. Such a person may accommodate to place distant objects in focus, but near objects will not be seen sharply without additional accommodation. The farsighted condition may be corrected by placing an eyeglass or lens having "positive refractive power" between the eye and the object with such lens causing light from distant objects to converge as the light passes through the lens. The light then passes through the lens of the eye to focus on the retina so that distant objects can be viewed effectively, without accommodation.
Astigmatism was referred to earlier as causing a distortion of the focusing of light passing through the lens of the eye. Astigmatism is a condition in which the first refracting surface of the eye, i.e., the cornea, has unequal curvature which prevents the focusing of light to a common point on the retina. Correction of this condition is accomplished by means of an eyeglass or lens having cylindrical curvature. Cylindrical curvature is that curvature represented by the side of a cylinder (as opposed to spherical curvature which is that represented by the surface of a sphere). Cylindrical and spherical lenses may be either positive or negative, with positive lenses being ones which are thicker in the middle than at the edge and negative lenses being ones which are thinner in the middle than at the edge. Positive and negative refractive lenses were mentioned above when describing correction of refractive errors in the eye. By orienting a negative or minus cylindrical lens of appropriate power so that its long axis (the axis perpendicular to the direction of maximum curvature) is overlying and parallel with the positive astigmatic axis of the eye (that axis perpendicular to the direction of greatest curvature of the front surface of the eye), astigmatism may be corrected. The effect of such a cylindrical lens is to perform refraction of light in a direction perpendicular to the axis of astigmatism and by an amount sufficient to compensate for the difference in curvature of the surface of the eye.
Lens power is the ability of a lens to refract light, i.e., to converge light if the lens is positive or to diverge light if the lens is negative. Lens power is measured in diopters, which is the reciprocal of the focal length of the lens, measured in meters. The focal length of a lens is defined as the distance from the lens to a point (for spherical lens) or line (for cylindrical lens) at which light converges after the light enters the lens in parallel and passes therethrough (for a positive lens) or from which the light appears to diverge after entering the lens and passing therethrough (for a negative lens). These definitions are well-known in the field of optics and opthalmology.
At the present time, eye examinations to determine the prescription of eyeglasses to correct nearsightedness, farsightedness and astigmatism are performed manually by ophthalmologists, optometrists, and technician refractionists. These examinations generally begin with some type of rough screening to determine generally if the eye is nearsighted or farsighted. A number of objective measurements may be utilized for this rough screening including retinoscopy. In retinoscopy, the Examiner makes a rough determination of the refractive error of the subject's eye by positioning a so-called trial lens (one or a number of lenses having different corrective powers used for eye examinations), introducing a slit of light into the subject's eye, moving the slit of light at right angles to the length of the slit, and observing how it is reflected from the retina of the eye. The Examiner is able to determine generally the refractive error of the eye by the way the reflected light moves as the slit of light is moved and by changing the power of the trial lens until certain conditions of reflected movement are met.
Another kind of rough screening may be performed by alternately placing medium power plus and minus spherical lenses before the eye, superimposed with a trial lens, as the subject views a displayed object or symbol. The subject's indication of which medium power spherical lens provides the sharper viewing of the symbol guides the Examiner in changing the trial lens to solicit another choice from the subject. For example, if a plus power trial lens is being used and the subject indicates a preference for the combination of the trial lens and the plus spherical lens, then the Examiner changes the trial lens to be slightly more positive and again queries the subject as to which combination of the trial lens and the plus and minus spherical lens is preferred. An approximation of the power necessary to correct the subject's refractive error is indicated by a reversal in the subject's choice of the plus spherical lens combination over the minus spherical lens combination as he compares the two. It will be recognized that in this type of rough screening, the subject is usually choosing between two rather blurred images. For this reason, only a rough approximation of the correct power can be made.
If the previous eyeglass prescription is available either in written form or from the eyeglasses themselves, this information may be used in place of performing the rough screening. This is especially true if the subject can see fairly well with such eyeglasses since then, only a small adjustment may be necessary to correct the refractive error.
Further refinement of the rough screening results is necessary if the subject is to see clearly. This refinement may be either so-called "subjective refinement" requiring a conscious response by the subject as to his preference of, for example, displayed symbols, or objective refinement in which no conscious response is required of the subject. In either type of test, the purpose is to determine which corrective lenses will maximize the subject's visual acuity, that is, his ability to discriminate and identify the shapes of symbols of certain sizes displayed at a certain distance from the subject. Visual acuity is usually designated by fractions such as 20/20, 20/30, etc., in which the numerator represents the distance between the subject and the displayed symbols and the denominator represents a measure of the size of a symbol barely discernable by the subject. This size is in terms of the distance which a normal subject could see the symbol. For example, 20/40 means that the subject could barely read a symbol at 20 feet which a person with normal vision (20/20) could read at 40 feet.
One type of objective refinement involves the measurement of the electroencephalograms of a subject as he views test symbols with different trial lens configurations. These "visually evoked responses" (VER) may then be examined to determine the visual acuity of the subject as a function of the amplitudes of the signals recorded on the electroencephalograms. When the visual acuity is a maximum, the signal amplitude will be maximum.
Another type of objective examination for testing visual acuity is known as optokinetic nystagmus (OKN). In this test, the reflex "following movement" of the eye is monitored as black and white vertical bars are moved horizontally across a screen in front of an eye. The eye of a subject with good visual acuity will, by reflex action, fix upon one of the bars, follow it until it becomes difficult or impossible to see, and then jerk quickly back to assume fixation on another bar, with the following rate of the eye matching the rate of bar movement. The visual acuity is inversely proportional to the width of the bars required for the "following movement" to be elicited. Thus, a subject with poor vision requires larger bars than does a patient with good vision to exhibit the appropriate following movement of the eye.
Even if an accurate final prescription for eyeglasses can be determined by one of the objective refinement tests, the subject may be so accustomed to accommodating in order to see clearly that eyeglasses which eliminate the need for such accommodation are undesirable to the subject. Subjective refinement enables the Examiner to determine an eyeglass prescription which will provide the subject with maximum comfort. This may be desirable even if some sacrifice in sharpness of the visual image must be suffered. Thus, subjective refinement is generally desirable and this type of testing is the most difficult and time-consuming part of an eye examination. Much patience is required on the part of the Examiner and persistent attention to detail on the part of the subject. If the subject feels rushed or gets bored, a hasty and incorrect decision may lead the Examiner in the wrong direction in presenting test lenses to the subject. Backtracking may thus be required, but even if it isn't, rechecking is often desirable to ensure the accuracy of the examination.
It is an object of the present invention, in view of the above-described methods for manually measuring refractive error, to provide an automatic refraction apparatus and method implemented by automatic data processing equipment in combination with test symbol projection apparatus and a trail lens system.
It is another object of the present invention to provide automatic apparatus and method for subjectively determining the refractive error of a subject rapidly and accurately.
Presently-used trial lens systems consist of a pair of rotatable turrets, each holding lenses of different power about the periphery thereof. The different trial lenses in one turret may be rotated into position in front of one of the subject's eye while the lenses in the other turret may be rotated into position in front of the other eye of the subject. The subject views test symbols through the lenses in the turrets and expresses, for each eye, a preference for one lens of each of successively presented pairs of lenses. This is usually done by simply rotating one lens of a pair in front of an eye then rotating the other lens of the pair in front of the eye and asking the subject to express his preference. As is evident from the above description, the power of the lenses positioned in front of the eye is varied by discrete "jumps" with manual rotation of the turret. Thus, the accuracy of the determination of the refractive error is dependent, in part, on the magnitude of the lens power increments which can be presented to the subject.
It might be noted here that some turrets include two or even three coaxial, contiguous elements, each of which holds a plurality of lenses of different power. Each element is independently rotatable so that each lens of each element may be effectively aligned with each lens of the other elements. In this manner, the many different combinations of lenses provide a fairly large number of different trial lens powers which may be presented to the subject; however, the trial lens power changes must still be made in discrete jumps.
Another problem with the currently used turret systems is that the number of lenses through which the subject is to view the test symbol can vary depending upon the positioning of the turret elements. For example, for one setting three lenses may be positioned before the subject whereas for another setting, only one or two lenses may be aligned because one of the turret elements is positioned so that only an opening (with no lens) in the element is aligned with the other lens or lenses. Of course, with variations of the number of lenses through which a test symbol is viewed, light transmission through the lens combinations varies and thus the relative brightness of the symbol varies. This may adversely influence the preferences expressed by the subject in choosing between the test symbols.
It is still another object of the present invention to provide automatic refraction apparatus and method having a continuously variable lens system through which the subject views a test symbol.
It is also an object of the present invention to provide apparatus and method for automatically controlling the variation in power of the lens system.
It is a further object of the present invention to provide method and apparatus in which periodic objective visual acuity tests are utilized as one determinant of the choice of lens system power to present to the subject.
It is still a further object of the present invention to provide such a lens system in which the number of lenses before an eye is maintained constant as the power of the lens system is varied.
If a test symbol of fixed size is presented on a screen for viewing by a subject, and the power of the lens system is varied, it will appear to the subject that the size of the symbol varies. Thus, when the subject is called upon to indicate a preference based on sharpness or clearness, between the test symbol viewed through the lens system of one power and the test symbol viewed through the lens system of a different power, the subject may be influenced by the size of the symbol. This is undesirable and even though the subject is cautioned against this, his preferences may still be influenced by the size of the symbol.
It is therefore an object of one aspect of the present invention to provide refraction apparatus and method for automatically varying the size of a test symbol presented for viewing by a subject to compensate for changes in power of the lens system through which the subject is viewing the symbol.
It is also an object of this aspect of the present invention to automatically control the magnification of test symbols so that they appear to be of constant size to a subject regardless of the variation in power of the lens system through which the subject is viewing the symbol.
As discussed above, in the course of an eye examination to determine refractive error using presently known techniques, a subject is called upon to indicate a preference between a test symbol viewed through one set of trial lens and the same test symbol viewed through a different set of trial lens. Preferences are solicited for successive pairs of lens combinations compared with the test symbol being presented at the same location on a screen. The examiner successively presents the trial lens of each pair to the subject and the subject then indicates a preference either for the "previous" symbol or the "present" symbol (or something similar to this). Because the subject must indicate a preference between what appears to be consecutively presented symbols at the same location on the test screen, confusion can arise in the course of the subject attempting to communicate his preferences to the examiner.
It is an object of another aspect of the present invention to provide apparatus and method for presenting test symbols alternately at two different locations for viewing by a subject to thereby enable the subject to indicate a preference by identifying the location of the "preferred" symbol.
It is an object of still another aspect of the present invention to provide apparatus and method for presenting test symbols simultaneously at two different locations for viewing by a subject.
It is also an object of the present invention to provide a manual response device by which a subject can identify at which of two locations a preferred test symbol is being presented.
It is still another object of the present invention to automatically monitor the direction of gaze of the subject and to change the lens power of the lens system when the direction of gaze changes from the first location to the second location.